With the transition from analog to digital TV, empty TV channels are now available for use by other wireless applications. These empty channels are called TV white space. The two most common means of protecting licensed services (incumbents) that operate in the TV band are: the use of geo-location information with database inquiry; and, TV white space spectrum sensing by TV band devices.
A TV band device that uses the geo-location information with database inquiry technique provides a TV white space database with its current location, and the database responds with a list of available TV white space channels that the TV band device may use. The TV band device then transmits on one or more of those channels. Although this method is simple and efficient, it has certain drawbacks. One drawback is that the geo-location information needs to be very accurate. If the geo-location information provided to the database is not precise, the database must be operated on a principle that the TV band device may be anywhere within an area indicated by a lowest level of specificity of the geo-location information provided. The database is therefore programmed to protect incumbents by returning conservative inquiry results, which compromises TV white space utility.
As an alternative, a TV band device may use spectrum sensing to locate unused TV white space channels. For example, in a recent FCC ruling for TV white space, sense-only TV band devices are permitted to rely exclusively on spectrum sensing to detect available TV white space channels. The sensitivity level is −114 dBm for ATSC and NTSC signals, and −107 dBm for low power auxiliary signals including wireless microphone signals. These sensitivity levels need to be achieved in the presence of an adjacent channel ATSC signal as strong as −28 dBm.
However, the sense-only mode has some fundamental technical drawbacks which make it difficult to implement. First, the sensitivity levels set by governing authorities are difficult to achieve, especially in the presence of high power adjacent channel signals. Second, while spectrum sensing can determine the existence of a signal in a TV channel, as well as the type of signal that was detected, spectrum sensing cannot tell if the signal originated from a licensed or an unlicensed source. Third, the sense-only mode does not provide a mechanism to permit a governing authority to exclude the TV band device from any particular channel, if required. Furthermore, there are many types of licensed signals in the TV band, and the spectrum sensing capability of a TV band device may only be capable of detecting certain ones of those licensed signal types, which leaves those undetectable types of licensed signals unprotected. There are also certain excluded areas, such as along the U.S./Canadian border region, and areas around radio astronomy receiver sites where TV band devices may not be permitted to utilize TV white space. As understood by those skilled in the art, it may prove difficult to use spectrum sensing to determine if a TV band device is within an excluded area.
It is also now being recognized that TV white space is not the only spectrum band that requires spectrum management. The problems discussed above also apply to any unlicensed band, including the 900 MHz, 3.5 GHz, 5 GHz, and 60 GHz bands.
There therefore exists a need for a system and method of implementing a cognitive radio device with enhanced spectrum sensing.